Cochlear implants (CI) help profoundly deaf or severely hearing impaired persons to perceive environmental sounds. Unlike conventional hearing aids, which just apply an amplified and modified sound signal, a cochlear implant is based on direct electrical stimulation of the auditory nerve. The intention of a cochlear implant is to electrically stimulate nervous structures in the inner ear so that hearing impressions most similar to normal hearing are obtained.
A cochlear implant system includes an external speech processor and the implanted stimulator. The speech processor contains a power supply and is used to perform signal processing of the acoustic signal to extract stimulation parameters for the implanted stimulator. The implanted stimulator generates stimulation patterns and conducts them to auditory nervous tissue by an electrode array which usually is positioned in the scala tympani in the inner ear. A connection between the speech processor and the implanted stimulator can be established by encoding digital information in an rf-channel and coupling the signal transcutaneously using an inductive coupled coils arrangement. The information is decoded within the implanted stimulator by envelope detection of the rf signal.
Applying a stimulus pulse to nerve tissue creates an action potential. The period of time immediately after the action potential is referred to as the refractory period, during which the nerve tissue has not returned to its resting state and is not ready to respond to a second stimulus. The interval during which a second action potential absolutely cannot be initiated, no matter how large the applied stimulus, is known as the absolute refractory period. The interval after the absolute refractory period is known as the relative refractory period during which second action potential is inhibited but not impossible. The relative refractory period ends once the nerve tissue is ready to respond to another stimulus with a normal action potential.
Stimulation strategies employing high-rate pulsatile stimuli in multi-channel electrode arrays have proven to be successful in giving high levels of speech recognition. One example is the “Continuous Interleaved Sampling (CIS)”—strategy, as described by Wilson et al., Better Speech Recognition With Cochlear Implants, Nature, vol. 352:236-238 (1991), which is incorporated herein by reference. For CIS, symmetrical biphasic current pulses are used, which are strictly non-overlapping in time. The rate per channel typically is higher than 800 pulses/sec. Other stimulation strategies may be based on simultaneous activation of electrode currents.
For high-rate pulsatile stimulation strategies, some patient specific parameters typically need to be determined. This is done some weeks after surgery in a fitting procedure. For given phase duration of stimulation pulses and for given stimulation rate, two key parameters to be determined for each stimulation channel include:                1. the minimum amplitude of biphasic current pulses necessary to elicit a hearing sensation (Threshold Level, or THL); and        2. the amplitude resulting in a hearing sensation at a comfortable level (Most Comfort Level, or MCL).For stimulation, only amplitudes between MCL and THL for each channel are used. The dynamic range between MCL and THL typically is between 6-12 dB. However, the absolute positions of MCLs and THLs vary considerably between patients, and differences can reach up to 40 dB. To cover these absolute variations, the overall dynamic range for stimulation in currently used implants typically is about 60 dB.        
There are several methods of setting the MCLs and THLs. For example, they can be estimated during the fitting procedure by applying stimulation pulses and asking the patient about his/her subjective impression. This method usually works without problems with post-lingually deaf patients. However, problems occur with pre-lingually or congenitally deaf patients, and in this group all ages—from small children to adults—are concerned. These patients are usually neither able to interpret nor to describe hearing impressions, and only rough estimations of MCLs and THLs based on behavioral methods are possible. Especially the situation of congenitally deaf small children needs to be mentioned here. An adequate acoustic input is extremely important for the infant's speech and hearing development, and this input in many cases can be provided with a properly fitted cochlear implant.
One approach for an objective measurement of MCLs and THLs is based on the measurement of the EAPs (Electrically Evoked Action Potentials), as described by Gantz et al., Intraoperative Measures of Electrically Evoked Auditory Nerve Compound Action Potentials, American Journal of Otology 15 (2):137-144 (1994), which is incorporated herein by reference. In this approach, the recording electrode is usually placed at the scala timpani of the inner ear. The overall response of the auditory nerve to an electrical stimulus is measured very close to the position of the nerve excitation. This neural response is caused by the super-position of single neural responses at the outside of the axon membranes. The amplitude of the EAP at the measurement position is between 10 μV and 1800 μV. Information about MCL and THL at a particular electrode position can first of all be expected from the so called “amplitude growth function,” as described by Brown et al., Electrically Evoked Whole Nerve Action Potentials In Ineraid Cochlear Implant Users: Responses To Different Stimulating Electrode Configurations And Comparison To Psychophysical Responses, Journal of Speech and Hearing Research, vol. 39:453-467 (June 1996), which is incorporated herein by reference. This function is the relation between the amplitude of the stimulation pulse and the peak-to-peak voltage of the EAP. Another interesting relation is the so called “recovery function” in which stimulation is achieved with two pulses with varying interpulse intervals. The recovery function as the relation of the amplitude of the second EAP and the interpulse interval allows conclusions to be drawn about the refractory properties and particular properties concerning the time resolution of the auditory nerve.
Besides cochlear implant systems as such, some subjects with some residual hearing (partial deafness) are now benefiting from combined electric and acoustic stimulation (EAS) such as was first described in von Ilberg et al., Electric-Acoustic Stimulation Of The Auditory System, ORL 61:334-340 (1999), which is incorporated herein by reference. EAS systems combine the use of a hearing aid (HA) device to provide acoustic-mechanical stimulation of lower audio frequencies to the subject's ear drum and a cochlear implant (CI) to provide intracochlear electrical stimulation of higher audio frequencies to the auditory nerve. For example, see Lorens et al., Outcomes Of Treatment Of Partial Deafness With Cochlear Implantation: A DUET Study, Laryngoscope, 2008 February: 118(2):288-94, which is incorporated herein by reference.